The tissue engineering field has created great demands for the next generation of biomaterials and biofabrication techniques. These materials and techniques need to provide simple and economical biomaterials that are cell instructive and suitable for transplantation. These materials must recreate and mimic the 3D in vivo niche and be able to provide the proper cues for the cells to migrate, proliferate, differentiate, promote angiogenesis, and ultimately direct the regeneration native tissue architecture. (Prestwich, Ghaly, Brudnicki, Ratliff, & Goligorsky, 2011).
Biomaterials generally fall into two categories, synthetic and naturally derived materials. Recent promising efforts have been made using primary cells in combination with synthetic matrices, such as PEG (Gilbert, et al., 2010), and poly(glycolic acid)(PGA)/poly-L-lactic acid (PLLA) (Levenberg, et al., 2005) (Boonen & Post, 2008) natural and synthetic hybrid materials such as collagen-poly(vinyl alcohol) (Abedi, Sotoudeh, Soleymani, Shafiee, Mortazavi, & Aflatoonian, 2010), naturally derived materials created from decellularization of native tissues (De Coppi, et al., 2006) and repolymerized naturally occurring extracellular matrix components such as collagen (Grefte, Kuijpers-Jagtman, Torensma, & Von Den Hoff, 2010; Yost, et al., 2004). These materials have been used somewhat successfully in the laboratory for cell culture, small and large animal in vivo testing, and to a very small extent human surgical transplantation. A common fabrication technique for these materials is the creation of the hydrogels. Hydrogels are polymeric materials that can retain large amounts of water without dissolving. There are several techniques commonly used to create these hydrogels which include chemical cross-linking, photo cross-linking, and sol-gel synthesis. Other hydrogels are formed through spinning, bioprinting or microfabrication process among other techniques (Prestwich, Ghaly, Brudnicki, Ratliff, & Goligorsky, 2011).
Tissue scaffolds plays a crucial role towards tissue regeneration process. The ideal scaffold has to fulfill several requirements such as the adequate composition, specific cell population, and well-defined architectural features. Scaffold created from natural biological materials provide ideal compatibility and functionality. Collagen is a natural and a major constituent of the extracellular matrix that contains the necessary biological information that directs the cell behavior (Yost, et al., 2004). Collagen Type I accounts for 70-90% of collagen in the body. Specific techniques need to be developed in order to reconstitute a collagen scaffold that will retain its mechanical properties and micro geometric features similar to the natural tissue, and induced in vivo cell behavior.
Novel 3D tubular collagen scaffold for cardiovascular tissue engineering have been successfully developed and commercialized using a counter rotating cone extrusion system, as disclosed in U.S. Pat. No. 7,338,517 of Yost, et al. and (Yost, et al., 2004), which are incorporated by reference herein. The device is capable of producing continuously spiraling alignment of the collagen fibers, which is seen in the layers of cardiac myocytes and extracellular matrix. This is an important key factor in vivo, because it provides the heart the ability to contract and move the blood from its ventricular cavity to the systematic vasculature. The collagen provided the biological information needed to direct cell behavior and along with the precise fiber orientation, the tube wall contracted spontaneously. The cardiac myocytes had developed an in vivo phenotype, possess aligned myofibers and developed sarcomeres. Furthermore, using this 3D tubular scaffold, Valarmathi et al. was able to induce the maturation and differentiation of BMSCs into vascular lineages, vasculogenesis, which was able to support microvessel morphogenesis.
One unique and unexpected aspect of this engineered scaffold is that it promoted the expression of an in vivo like phenotype and tissue organization for many cell types. (Yost, et al., 2004; Evans, Sweet, Price, Yost, & Goodwin, 2003; Valarmathi, Yost, Goodwin, & Pott, 2008; Goodwin R. L., Nesbitt, Price, Well, Yost, & Potts, 2005). The tube scaffold has been used to study embryonic and (Yost, et al., 2004; Goodwin R. L., Nesbitt, Price, Wells, Yost, & Potts, 2005) neonatal cardiomyocytes, cardiac fibroblasts, coronary vasculogenesis, and development of tissue-engineered cardiac valves (Evans, Sweet, Price, Yost, & Goodwin, 2003; Yost, Franchini, Goodwin, Nesbitt, & Price, 2005). We have also used our scaffold to regenerate skeletal muscle in a rat hernia model (Fann, Terracio, Yan, Franchini, & Yost, 2006). The tube has been combined with satellite cells and used to study angiogenesis and inflammation on a molecular level in vivo during skeletal muscle repair (Propst, et al., 2009).
During the course of these investigations, it was realized that the next generation of fabrication technology was needed to create collagen scaffold with branch points and more intricate, in vivo like geometries. The approach mimics early morphogenesis, based on the realization that both genes and physical forces regulate self-assembly and 3-dimensional pattern formation of complete tissues.
Not only is the 3D structure necessary for vasculogenesis but so is the composition of the substrate. Previous studies have shown that the formation of endothelium-lined tubular structures was promoted in vitro by the presence of laminin in the matrix (Madri, Patt, & Tucker, 1988). In a pure collagen type I, there was a delay in the promotion of endothelial cells to enhance tubulogeneis (Madri, Patt, & Tucker, 1988).
Although this technology provides the ability to re-create the in vivo architecture and vasculogenesis of BMSCs in the extracellular matrix, it has some limitations: specifically, it doesn't replicate the complex geometries and architectures found in in vivo tissues. The interaction of the cells with a specific arrangement and structure of the in vivo niche are vital to provide the necessary cues for the development of in vivo phenotypes. (Yost, et al., 2004).
As such, a need exists for methods of re-creating the in vivo architecture and vasculogenesis of BMSCs in the extracellular matrix while replicating the complex geometries and architectures found in in vivo tissues. For example, the next generation of fabrication technology is currently needed to create a collagen scaffold with branch points and more intricate, in vivo like-geometries.